The present invention relates to an impact printer with a print head grounded in such as way as to reduce electrocorrosion.
The print head of an impact printer has a plurality of electromagnets embedded in a filling compound that provides electrical insulation and serves as a heat sink. To prevent build-up of static charges generated during paper transport, the print head is electrically grounded to the printer's chassis, that is, to the printer's frame ground.
Epoxy resin compounds are often used as the filler in the print head, but at temperatures above 150.degree. C., which are not unknown in high-speed impact printing, these compounds begin to dissociate, liberating chemically reactive negative ions. In addition, the insulation resistance of the compound is reduced and current begins to leak from the positive poles of the electromagnets to the frame ground. As part of this current, the negative ions migrate toward the positive poles of the electromagnets. At high temperatures these ions react readily with the insulation protecting the wiring of the electromagnet coils, and with the conductive metal of the wiring itself. Over time these reactions, referred to hereinafter as electrocorrosion, can produce short circuits or open circuits in the electromagnet coils, ending the useful service life of the print head.
Electrocorrosion increases with the operating temperature and operating voltage of the print head. In recent high-speed impact printers, electrocorrosion has become a serious problem.
To avoid electrocorrosion, some printer manufacturers have turned to silicon-based resin filling compounds. After curing, these compounds remain chemically stable even at temperatures of 180.degree. C., so electrocorrosion is effectively prevented. Before curing, however, the theological properties of these compounds leave much to be desired. Particularly in high-pin-count print heads such as twenty-four-pin heads, the high viscosity of the silicon-based resin makes it difficult to fill the narrow spaces between the closely-packed electromagnets, so that the coils may be left partially exposed. The filling compound thus provides inadequate heat sinking, resulting in early burn-out of the exposed coils.
For densely-structured impact print heads, no alternative is readily at hand to epoxy resin filling compounds, electrocorrosion notwithstanding.